The expression ASI network (ASI: Actuator Sensor Interface) refers to a network with a bus system for communication between a controller (master) and field devices (slaves). An ASI bus system is a bit-oriented fieldbus for automation, and is optimized for connection of simple sensors, actuators and field devices which interchange only a small number of information bits with the controller and can not only carry out data communication but can also be supplied with electrical power via a generally 2-core line.
In order to allow electrical field devices to be operated in an explosion-hazard area, devices are required which have a specific type of ignition protection in order that they do not initiate any risk of ignition. Field devices which are intrinsically safe are particularly advantageous, since no additional measures are required to protect the fieldbus, for example encapsulation and shielding measures, in order to ensure explosion-proof operation.
With a fieldbus system such as this, it is generally possible to connect a large number of field devices in parallel to the bus lines. However, despite the field devices for the fieldbus being designed to be intrinsically safe in the explosion-hazard area, it is necessary for the power supplied via the bus lines to be limited in order to ensure that a spark which could initiate ignition cannot occur anywhere or at any time even in the bus line fault conditions stipulated in the standards. However, the energy provided by the controller cannot be reduced indefinitely, because of the transmission paths (line lengths) required in the fieldbus system and the minimum power required by the individual fieldbus devices.
Known fieldbus systems which are used in an explosion-hazard area and make use of the “intrinsically safe” type of ignition protection therefore restrict the length of the bus lines of the fieldbus to a specific distance (for example to 1 km), and restrict the number of fieldbus devices that can be connected to about 10, because it is not possible to supply a greater number of field devices, because of the limited power fed in.